1. Field of the Invention
The invention relates to a digital transmission system comprising a receiver, to receiver including an equalizer for estimating transmitted binary symbols from a sequence of sample values of a received signal distorted by a transmission channel, by implementing a reduced-state sequence estimation method or reduced-state single symbol estimation method.
Furthermore, the invention relates to a receiver which includes an equalizer for estimating transmitted binary symbols from a sequence of sample values of a received signal distorted by a transmission channel, by implementing a reduced-state sequence estimation method or reduced-state single symbol estimation method.
2. Description of the Related Art
Such receivers are used, for example, in digital mobile radio according to the GSM standard. According to the GSM standard, digital signals are transmitted in a TDMA method by a GMSK modulation. The data transmission is then influenced by a time-variant transmission channel. More particularly, multipath propagation and reflections determine differences of delay and phase shifts for the transmitted digital data symbols in the received signal and lead to a superpositioning of adjacent data symbols. The fact that a received signal for a data symbol, is influenced by d previous data symbols is known as intersymbol interference (ISI). Then d is an integer defining the memory depth of the transmission channel.
For the equalization of the received signal which is linearly distorted as a result of multipath propagation and transmitting-end and receiving-end band limitation filters (intrinsic impulse noise with linear demodulation of the GMSK signal), the receiver is to be adapted, for data reconstruction, to the respective time-variant transmission properties of the transmission channel. Therefore, an estimation is made of the respective impulse response of the currently distorting transmission system, this system comprising not only the transmission channel, but also the influences of the GMSK modulation and a receiver input stage which produces sample values of the received digital signal. For this purpose, a substitute system describing the transmission system is formed, with the aid of which, estimated impulse response data are estimated according to the Maximum Likelihood Sequence Estimation (MLSE) method via the execution of a Viterbi algorithm, more particularly, a soft-output Viterbi algorithm, or a single symbol estimation method.
With this method, the most probable transmit sequence is determined from all possible data sequences, while taking into account the received sequence and the estimated impulse response of the transmission system. More particularly, the Viterbi algorithm is suitable for estimating the data symbols according to the MLSE method. The Viterbi algorithm is known from "The Viterbi algorithm", G. D. Forney, Jr., IEEE Proceedings, vol. 61, pp. 268-278, 1973. Additional information to the hard-decision estimates of data symbols is produced by the Soft-Output Viterbi Algorithm, which is known, for example, from "A Viterbi algorithm with soft-decision outputs and its applications", J. Hagenauer and P. Hoher, Proceedings of the GLOBECOM 1989, pp. 47.1.1-47.1.7, Dallas, 1989. With single symbol estimation, optimum maximum a-posteriori symbol-by-symbol decoder algorithms are used according to "Optimal decoding of linear codes for minimizing symbol error rate", L. R. Bahl, J. Cocke, F. Jelinek, and J. Raviv, IEEE Transactions on Information Theory, IT-20: pp. 284-287, 1974, or modification of this algorithm, respectively, found in "Optimum and Sub-Optimum Detection of Coded Data Disturbed by Time-Varying Intersymbol Interference", W. Koch and A. Baier, Proceedings of the GLOBECOM 1990, pp. 807.5.1-807.5.6, San Diego, December 1990. With an equal value of the estimation of the received signals, the manufacturing costs of the equalizer in a first approximation proportionally rise by 2.sup.d i.e., they rise exponentially with the depth d of the memory of the transmission channel. In a sequence estimation method with a complete or reduced number of states, the substitute system actuated by binary symbols, this substitute system describing the effect of signal distortions as a result of multipath propagation and the effect of intrinsic impulse noise, is considered a so-called finite state machine or a trellis coder respectively, in whose associated trellis diagram, the binary symbol sequences are represented as paths. This is described, for example, in "Trelliscodierung in der digitalen Ubertragungstechnik--Grundlagen und Anwendungen", J. Huber, Springer Verlag, Berlin 1992. The object of the sequence estimation method is to determine, on the basis of a sequence of sample values of the received signal, the binary symbol sequence transmitted most probably. The implementation expenditure necessary for this purpose is proportional to the number of memory states of the finite-state machine, this number rising exponentially with the degree of the substitute system and thus, with the maximum delay difference in the multipath propagation of the signal. For sequence estimation methods having a reduced number of states, the number of states is subdivided into classes for which each state class forms a so-called hyperstate with respect to the sequence estimation method. The reduction of expenditure is then that the sequence estimation method is executed only more in the Trellis diagram for the hyperstates. Embodiments for this are contained in, for example, "Trelliscodierung in der digitalen Ubertragungstechnik--Grundlagen und Anwendungen", J. Huber, Springer Verlag, Berlin, 1992. Together with the decisions which path to a hyperstate has maximum probability, the actual state of the trellis coder, the so-termed sub-state in a hyperstate, is determined at the same time. A correct determination of the probabilities for the next steps in the trellis diagram, the so-termed metric, becomes possible in this manner. It is advantageous to combine the states to a hyperstate in which the binary symbols stored in the substitute system are only different in the last d-r; 0.ltoreq.r&lt;d register cells, where d indicates the degree of the substitute system. In this manner, the number of states is reduced from 2.sup.d to 2.sup.r hyperstates, while each hyperstate includes 2.sup.d-r sub-states. Thus the expenditure as against a sequence estimation with the complete number of states is reduced by the factor 2.sup.d-r.
European Patent Application corresponding to U.S. Pat. No. 5,307,374 discloses EP-0 488 456 A2 a maximum likelihood receiver which also uses a sequence estimation with a reduced number of states in the equalizer. The variance of the estimation error is then increased for an estimated sequence of binary symbols, because a shortened impulse response of the substitute system is used for the sequence estimation. Since a smaller number of coefficients is available when a sequence is to be estimated with a reduced number of states for the path distinction in the trellis diagram, the reliability of the sequence estimation is reduced. More particularly, when the number of states is reduced to a degree necessary for a suitable reduction of the manufacturing costs, the reliability of the sequence estimation becomes inadequate.
The publication DE-Z: G. Zimmermann and W. Rupprecht: "Application of a Simplified Fano Metric to Adaptive Receivers for Digital Mobile Radio Systems", ETT. Vol. 5, No. 3, May-June 1994, pp. 65-70 discloses a receiver which includes an adaptive Whitened Matched Filter (WMF). This WMF is realized in a modified Kalman filter. After the coefficients of the channel impulse response have been determined by the equalizer, a new minimum-phase overall channel impulse response is generated by a convolution in the time domain or a multiplication in the frequency domain by the impulse response of a respectively adapted Kalman filter. This basic impulse response estimated from the so-called midamble, however, changes during the burst in response to the channel properties, so that the impulse response can neither be assumed to be known at the start nor at the end of the time slot. When the equalization is divided into forward and backward detection, the basic impulse response then has a minimum phase for the forward search in an extreme condition, whereas, a maximum-phase channel impulse response occurs during the backward search. In a reduced-state sequence estimation, this leads to erroneous detection. When equalization takes place in one direction only, that is, from the start of the burst to the end of the burst via the basic impulse response estimated on the basis of the midamble, and the basic impulse response at the start of the burst is not adapted to the unknown impulse response by means of iterative adjustment, this method also calls forth degradation.